1. Field of the Invention
The invention relates to a plasma potential measuring method and apparatus, and more particularly to a plasma potential measuring method and apparatus that allows reduction in error factor. The invention also relates to a plasma potential measuring probe, and more particularly to a plasma potential measuring probe that allows accurate measurement in ion current.
2. Description of the Related Art
The emissive probe method is known as a method that allows the measurement of plasma potential in plasma atmosphere without any analysis.
FIG. 1 is a system diagram showing a plasma potential measuring apparatus using the known emissive probe method. As shown in FIG. 1, the emissive probe method is conducted such that a filament 101 in plasma atmosphere is heated by the application of current from a heating power source 102, a potential applied to the entire filament 101 is varied by a power source 103, and the plasma potential is determined to be a potential applied when no current I is flown into the filament 101 (i.e., in floating probe state) [for example, Kanji Fujita, “Plasma Potential Measurement by Emissive Probe”, J. of Phys. Soc. of Jpn., vol. 39, No. 5(1984) 366-369].
The potential (floating potential) of floating probe without thermoelectronic emission is, as compared to plasma potential, reduced by a level (sheath voltage) corresponding to the energy of electron stream such that it satisfies the floating condition to block the electron stream from plasma. If thermoelectron is emitted from the floating probe, the stream of thermoelectron is in a direction to cancel the abovementioned electron stream and therefore, the sheath voltage lowers and the floating potential comes close to the plasma potential. If the floating potential exceeds the plasma potential, thermoelectron is not emitted.
However, the emissive probe method has some problems Namely, due to some error factors thereof, it is difficult to conduct the accurate measurement. Since the amount of thermoelectron generated varies depending on the temperature of filament 101, error in plasma potential measurement must be occurred depending on the filament temperature. Further, the filament 101 significantly wastes with time, and therefore the repeatability in measurement will deteriorate.